But work now to protect food security, reshape water sharing agreements and cut risks from worsening weather disasters could play a huge role in reducing future conflict and instability, they said in a report commissioned by G7 governments.

Both at-risk and stable countries would benefit, as they attempt to deal with problems such as uncontrolled migration, rising emergency relief bills, and demands for military assistance in conflict zones, the report said.

"The scale of security risks we're talking about is potentially enormous," said Dan Smith, a co-author of the report and head of International Alert, a UK-based peacebuilding organisation.

The report termed climate change "the ultimate threat multiplier", and said it should be a top foreign policy priority for the Group of Seven major industrialised democracies.

As food and water security worsen in many fragile parts of the world, "you can see the climate thread" in social upheaval from Egypt's revolution in 2011 to the rise of Boko Haram in Nigeria, Smith said at a discussion on the report at Britain's Foreign and Commonwealth Office this week.

SYRIA DROUGHT

The start of Syria's crisis was preceded by a brutal five-year drought in its main northeastern food-producing region, the report said.

The loss of crops and animals pushed many rural families to already overcrowded cities, increasing unemployment, it added.

Lukas Ruttinger, an author of the report from Adelphi, a German policy thinktank, emphasised that drought was not the main reason for Syria's crisis.

"We're not saying climate change caused the conflict in Syria. But it combined with other pressures that a repressive and non-responsive government was unable to manage," he said.

In Asia, Thailand's severe 2011 floods, which affected 2 million people, came on the heels of years of anti-government protests. After the disaster many people complained that state compensation had been unfairly distributed - and the government eventually fell in a 2014 coup, Ruttinger said.

Looking ahead, regions from the increasingly water-short Indus River basin in India and Pakistan to states already afflicted by conflict and poverty, such as Somalia, Afghanistan and Niger, will probably face some of the biggest risks of instability, the report noted.

In all of them, "we have to think about this in terms of managing risk, not solving the problem", Smith said.

The good news, he added, is that many of the elements of what needs to be done are starting to fall into place. Efforts to coordinate climate change adaptation, aid and peace-building efforts are growing, though they are "not systematic", he said.

Maintaining a distinction between financing for climate change and financing for development "is misleading and potentially dangerous", he warned, saying climate and development action must be integrated to be effective.

But many poorer countries want to keep the two types of aid separate to ensure that rich-country promises to mobilise $100 billion a year in international climate funding - on top of existing aid flows - are met, the experts said.

Insurance could play some role in reducing risks and providing stabilising payouts to disaster-hit families, Smith said. But the cost of providing insurance depends on analysing risks based on long-term trends, and climate change is bringing "profound disruption of existing trends", he said.

That could make the cost of providing insurance cover for some climate risks excessive, the experts said.

CREATIVE THINKING

Innovative thinking could help. A project to negotiate open border agreements for drought-hit nomadic herders who move from country to country in Africa's Sahel may ease pressures in the region's fragile states, said Baroness Joyce Anelay, a minister at the Foreign and Commonwealth Office.

The project, by French aid group Acting for Life, is supported by Britain's Department for International Development through its Building Resilience and Adaptation to Climate Extremes and Disasters (BRACED) programme.

On a broader scale, reducing climate-related security risks will require many changes, including better global risk assessments and support for food security through measures to build stocks and curb price fluctuations, experts said.

Improving local abilities to cope with climate stresses and finding ways to defuse water disputes between neighbouring nations will also be important, the report said.

Water sharing across national borders has in the past been a shining example of how to build cooperation and head off disputes, Smith said.

But with populations growing and demand for water rising as climate change in many cases cuts flows, a process for renegotiating water deals in line with those shifts is needed, he said.

Trying to reduce as far as possible the pressures driving world instability is crucial, Ruttinger said, because "we are already at the limit of what we can manage".

Our global editorial team of more than 50 journalists and about 250 freelancers covers the world’s under-reported stories at the heart of aid, development, women’s and LGBT+ rights, human trafficking, property rights, and climate change.